For example, a liquid material vaporization and supply apparatus for supplying a liquid raw material to semiconductor manufacturing equipment utilizing Metal Organic Chemical Vapor Deposition method (MOCVD method) is conventionally proposed (Patent Documents 1 to 3, for example).
The liquid material vaporization and supply apparatus of this kind heats and vaporizes a liquid raw material such as TEOS (Tetraethyl orthosilicate) in a vaporization chamber and supplies the vaporized gas to the semiconductor manufacturing equipment at a predetermined flow rate controlled by a flow rate control devise. Here, a liquid level needs to be controlled by detecting the liquid raw material level so that the amount of the liquid raw material reduced due to the vaporization can be supplied.
For example, a pressure-detection-type liquid level detection device detecting a reduction of a liquid raw material due to vaporization in a vaporizer by monitoring a pressure reduction in the vaporizer (such as Patent Document 2) and a thermal-type liquid level detection device utilizing a difference in heat dissipation constants between a liquid phase and a gas phase (such as Patent Documents 4 to 6) are known as tools for detecting a liquid level of a liquid raw material.
In the thermal-type liquid level detection device of this kind, as illustrated in FIG. 8, two protecting tubes P with respectively enclosed resistive temperature detectors R1 and R2 made of platinum or the like are vertically inserted into a container T, and a comparatively large constant current I1 (heating current) is supplied to the resistive temperature detector R1 for keeping a temperature of the resistive temperature detector R1 higher than an ambient temperature by the self-heating while a micro constant current I2 (current for measuring the ambient temperature) which only generates ignorable heat and is little enough for measuring the ambient temperature is supplied to the resistive temperature detector R2.
Then, the resistive temperature detector R1 to which the large constant current has been supplied generates heat. Here, the temperature of the resistive temperature detector in a gas phase G is higher than the temperature of the resistive temperature detector in a liquid phase L because the heat dissipation constant when the resistive temperature detector is in the liquid phase L is larger than the heat dissipation constant when the resistive temperature detector is in the gas phase G.
This means that a resistance value of a resistive temperature detector is larger when the resistive temperature detector is in the gas phase than when the resistive temperature detector is in the liquid phase. Therefore, it can be determined if the resistive temperature detector is located above or below the liquid surface by measuring a difference (absolute value) between a voltage output of the resistive temperature detector R1 to which the larger current has been supplied and a voltage output of the resistive temperature detector R2 to which the micro current has been supplied. In other words, it can be determined that the resistive temperature detector is located below the liquid surface in case the difference is smaller and the resistive temperature detector is located above the liquid surface in case the difference is larger.
FIG. 9 illustrates an example of a liquid level detection circuit and the constant currents are respectively supplied from a power source Vcc to the resistive temperature detectors R1 and R2 via constant current circuits S1 and S2. The constant current circuit S1 is configured to have a current larger than a current flowing through the constant current circuit S2 flow therethrough so that the micro current which is weak enough to generate the ignorable heat and allows the ambient temperature measurement flows through the resistive temperature detector R2 and the comparatively larger current of a current value larger than that of the current flowing through the resistive temperature detector R2 which heats the resistive temperature detector R1 to the high temperature. A terminal voltage V1 of the resistive temperature detector R1 and a terminal voltage V2 of the resistive temperature detector R2 are respectively inputted to an inversion input and a non-inversion input of a differential amplification circuit D and then a voltage signal corresponding to a voltage difference (V1−V2) between the terminal voltage V1 and the terminal voltage V2 is inputted to a comparator C. The comparator C compares the voltage difference (V1−V2) to a reference voltage V3 predetermined by dividing resistors R3 and R4.
When the resistive temperature detector R1 is in the liquid phase, a temperature rise of the resistive temperature detector R1 relative to the ambient temperature is smaller than the temperature rise when the resistive temperature detector R1 is in the gas phase. As a result, the output voltage from the differential amplification circuit D corresponding to the difference between the voltage signal outputted from the resistive temperature detector R1 and the voltage signal corresponding to the ambient temperature outputted from the resistive temperature detector R2 which is also in the liquid phase becomes lower than the reference voltage, and the output from the comparator C accordingly goes to a low level. On the other hand, when the liquid level drops and the resistive temperature detector R1 is exposed in the gas phase, the temperature rise relative to the ambient temperature becomes equivalent to the temperature rise of the resistive temperature detector R1 in the gas phase. Thus, the output voltage from the differential amplification circuit D corresponding to the difference between the voltage signal outputted from the resistive temperature detector R1 and the voltage signal corresponding to the ambient temperature outputted from the resistive temperature detector R2 which is also in the gas phase becomes higher than the reference voltage, and the output from the comparator C accordingly reaches a high level. Therefore, it is determined that the resistive temperature detectors R1 and R2 are in the gas phase when the output from the comparator C is the high level and that the resistive temperature detectors R1 and R2 are in the liquid phase when the output from the comparator C is the low level.
By measuring the terminal voltages V1 and V2, resistance values of the resistive temperature detector R1 and R2 can be calculated from the current values I1 and I2 by applying Ohm's law, and then the temperatures of the resistive temperature detectors R1 and R2 can be derived from the resistance values of the resistive temperature detectors R1 and R2 in case resistance change rates against the temperatures of the resistive temperature detectors R1 and R2 are known. Therefore, in the liquid level detection circuit, instead of by comparing the voltage outputs from the resistive temperature detectors R1 and R2, the liquid level determination can be done by comparing the resistance values of the resistive temperature detectors R1 and R2, or by calculating the temperatures of the resistive temperature detectors R1 and R2 from the respective resistance values using the resistance change rates against the temperatures of the resistive temperature detectors R1 and R2 and then comparing the calculated temperatures.